1. Field of the Invention
The present invention relates to a calibration reference light source and a calibration system using the same, which enable a user side to recalibrate wavelengths and sensitivities of a spectrophotometer and a spectral illuminometer.
2. Description of the Background Art
The spectrophotometer and spectral illuminometer are widely used to measure and evaluate radiances and colors of various light sources and display devices and normally have a construction as shown in FIG. 8. In this spectrophotometer, a light 201 to be measured is gathered by an objective optical system 202 to be incident on an incident slit 211 of a polychrometer 210. The beam incident on the slit forms a wavelength dispersed image of the incident slit 211 on a sensor array 214 via a diffraction grating 212 and an imaging optical system 213. Pixel intensity distributions En (n is a pixel number of 1, 2, . . . ) of the sensor array 214 according to the spectral intensity of the incident light are sent to a controller 204 via a processing circuit 203 to be converted into spectral radiances L(λ).
Calibrations of such a spectrophotometer includes a wavelength calibration and a sensitivity calibration. The wavelength calibration is performed by saving a correspondence table (n-λn correspondence table) of the pixel number n of the respective pixels of the sensor array 214 and a centroid wavelength λn. The pixel intensity distributions En in an actual light to be measured are converted into spectral intensity distributions E(λ) based on the above n-λn correspondence table. Further, the sensitivity calibration is performed by saving a ratio of a spectral radiance reference value L0(λ) to the spectral intensity distribution E0(λ) in the case where a reference light source including a known black body radiation source that is obtained a spectral radiance reference value L0(λ) such as an A-light source (2856 K) is measured , as a correction coefficient C0(λ). In other words,C0(λ)=L0(λ)/E0(λ)  (1).
At the time of a light measurement by the thus calibrated spectrophotometer, the spectral intensity distributions E(λ) obtained from the pixel intensity distributions En in accordance with the above n-λn correspondence table are converted into spectral radiances L(λ) as below using the above calibration coefficient C0(λ).L(λ)=C0(λ)·E(λ)  (2)
Even if being calibrated as above at the time of manufacturing, the spectrophotometer cannot avoid a sensitivity change due to a wavelength change caused by a change in the optical arrangement of the built-in polychrometer 210 and characteristic changes of optical elements and circuit elements such as the objective optical system 202 and the diffraction grating 212. Thus, recalibration (correction) needs to be performed at a sufficient frequency in order to maintain accuracy over a long term. However, a radiation exitance M(λ, T) of the A-light source, which has been conventionally used in the sensitivity calibration of the spectrophotometer, is given by Planck's law of radiation using C1, C2 as constants:M(λ, T)=C1·λ−5·exp[1−C2/(λ·T)]  (3)and relatively and absolutely largely depends on color temperature T.
Accordingly, it is necessary to control or monitor the color temperature to use this A-light source as a calibration reference. In order to maintain a radiation intensity of ±1% of the A-light source having a color temperature of 2856 K at 400 nm, the color temperature needs to be controlled with an accuracy of ±2 K. Thus, sufficient attentions need to be paid to the driving of the A-light source such as stabilization of a power supply voltage. On the other hand, at least two monitoring sensors having different spectral sensitivities are necessary for the monitoring of the color temperature. Filters, which are normally unavoidably temperature-dependent, are provided in these sensors, and wavelength errors of the filters need to be 0.5 nm or shorter. Further, attentions need to be paid to changes of the A-light source and the filters with time (e.g. wear damage of a filament in the A-light source).
Due to this difficulty to handle, the sensitivity recalibration (sensitivity correction) of the spectrophotometer is frequently performed by returning the spectrophotometer to a factory or service deposits of a manufacturer. Thus, it takes cost and time to return and recalibrate and it is difficult to recalibrate at a frequency necessary to maintain the accuracy. Accordingly, the present inventors previously proposed a calibration reference light source constructed by a plurality of single-wavelength light sources with stable wavelengths and semiconductor monitoring sensors with stable characteristics in Japanese Unexamined Patent Publication No. 2006-177785 (D1). Thus, this calibration reference light source using no A-light source is easy to handle and calibration at the user side is possible.
The wavelength stability of the single-wavelength light sources is essential even in the above prior art, relatively high-cost wavelength stabilized lasers such as gas lasers and temperature-controlled semiconductor lasers need to be used as the single-wavelength light sources. Further, lasers have a limit in usable wavelengths, single-wavelength reference lights of only about three wavelengths (408 nm, 532 nm and 635 nm) can be obtained within a visible range (380 to 780 nm) which is a general measurement wavelength range of spectrophotometers. The accuracy of correction coefficients at all the measurement wavelengths estimated based on data on the correction coefficients at these few wavelengths by interpolation or the like is low.